The present invention relates generally to a method for determining in-situ formation properties. More particularly, the present invention provides a method for estimating in-situ formation properties employing empirically derived systematics relating selected formation properties including compressional, bulk, and shear velocities with formation porosity and mineralogy content.
In the continuing search for subsurface minerals, including hydrocarbons, determination of in-situ formation properties has always been a goal of explorationists as an aid in interpreting formation lithology and pore fluid content. Until recently, numerous borehole logging techniques have been developed to directly gather information regarding in-situ formation properties of the earth adjacent the borehole. Exemplary of such techniques are borehole compensated sonic logging for determining formation compressional velocity, full wavetrain sonic logging for determining both compressional velocity and shear velocity, neutron and gravity logging for determining formation density and porosity as well as gamma-gamma and electric logging for determining density as well as porosity and lithology. More recently, others such as Castagna, J. P. et al., in "Relationships Between Compressional Wave and Shear-Wave Velocities in Clastic Silicate Rocks," Geophysics, Vol. 50, No. 4 (April 1985), pp. 571-581 have developed systematic models between compressional and shear velocities with formation properties. Castagna et al. describes relationships for general rock types (for example, clastic silicate rocks) derived from both literature and laboratory measurements for a wide variety of such rocks. Additionally, Ruhovets in U.S. Pat. No. 4,584,874 has described a method for determining porosity, clay content and mode of distribution in gas- and oil-bearing shaly sand reservoirs employing a neutron density log crossplot technique.
In the ongoing search for subsurface mineral deposits, determination of in-situ formation properties, including for example, compressional velocity V.sub.p, shear velocity V.sub.s, porosity .phi., and mineralogy content M, are of specific interest to explorationists. Indirect methods of determining in-situ formation properties are especially useful since direct measures thereof are not always obtained for every well, and in selected areas direct measures of certain formation properties cannot be obtained. For mixed lithologies such as sand/shale sequences in the sedimentary basins of the Gulf of Mexico, determination of in-situ formation properties can be especially enlightening to further evaluate potential mineral deposits. However, because both pore fluid and effective pressures can vary in such mixed lithologies, indirect methods for evaluating potential mineral deposits can produce erroneous results. In fact, there is a great paucity of measured velocity data on Gulf of Mexico formation rocks. This is especially so for the clay-rich sediments where formation shear velocities are "slow," i.e., formation shear velocities are less than compressional velocities in the borehole fluid. However, shear wave velocity measurements are of increasing importance in light of the potential for evaluating bright spots and offset dependent amplitude variations. Additionally, more and more exploration activities are being conducted in overpressured formations, and, as such, one must understand the effects of overpressuring on formation velocities.
Responsive to the ongoing need of explorationists to obtain the most information possible regarding in-situ formation properties, a novel method has been developed for estimating in-situ formation properties at a remote exploratory well employing empirically derived systematic relations of selected formation properties.